Medical devices are implanted in human bodies for monitoring physiological conditions, diagnosing diseases, treating diseases, or restoring functions of organs or tissues. Examples of such implantable medical devices include cardiac rhythm management (CRM) devices, neural stimulators, neuromuscular stimulators, drug delivery devices, and biological therapy devices. One particular example of implantable medical devices is a CRM device implanted in a patient to treat irregular or other abnormal cardiac rhythms by delivering electrical pulses to the patient's heart. Such rhythms result in diminished blood circulation. Examples of implantable CRM devices include pacemakers and defibrillators. Pacemakers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly or irregularly. Such pacemakers may coordinate atrial and ventricular contractions to improve the heart's pumping efficiency. Defibrillators are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators may also include cardioverters, which synchronize the delivery of such stimuli to portions of sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. In addition to pacemakers and defibrillators, implantable CRM systems also include, among other things, pacer/defibrillators that combine the functions of pacemakers and defibrillators, drug delivery devices, and any other implantable systems or devices for diagnosing or treating cardiac arrhythmias.
Batteries are used as energy sources for implantable medical devices. While the use of battery allows a medical device to be totally implantable, without the need of transcutaneous power transmission, the power consumption and the longevity of the medical device is limited by the capacity of the battery. For example, most treatments using implantable CRM devices are long-term treatments that may last up to the patient's lifetime. When the battery of an implantable CRM device is no longer able to provide sufficient energy for the operation of the device, the device is to be explanted and replaced. Thus, for patient safety and device longevity, among other reasons, there is a need for monitoring the status of a battery in an implantable medical device.